Indoor radon ( ^<222>Rn) concentrations were measured so as to understand the behavior of radon in wooden houses. The number of measured houses was 28 buildings which were located over a wide region from Hokkaido to Kagoshima. The measurement had been carried out for one to three years.The radon concentrations in wooden houses ranged from 3 to 22 Bq/m^3, and those in reinforced concrete houses were from 25 to 46 Bq/m^3 which were about 1.5 times as much as those in the wooden houses. In reinforced concrete buildings, the radon concentrations varied over a range of 21 to 1116 Bq/m^3. The large concentrations are due to radon emanation from many inorganic materials, and from the effect of small ventilation rates. In reinforced concrete buildings and under the floor of wooden houses, the radon concentrations were large from spring to summer. It was made clear that radon emanating from the soil under the floors enters the wooden houses. Therefore, the radon emanation could be reduced by covering the ground under the floors of the wooden houses with concrete or vinyl sheet.In the future, the wooden houses in Japan will become airtight one and the inorganic materials will be used much more, then the radon concentrations will be larger than nowadays like in the reinforced concrete buildings at this time.2).木造住宅では、床下及び地下室の濃度が高く300Bq/m^3を越える場合がある。そして、1階、2階と上階へいくに従って低くなる。このことから、木造住宅の最大のラドン発生源は床下地面であり、それは居室へと拡散していく。3).床下ラドン濃度は、一般に冬に低く、高温多湿となる夏期に高い。4).床下ラドン濃度は、床下地面の防湿施工によって低下する。5).床下換気口の適切な配置は床下ラドン濃度を低下させる。6).室内の空気が滞溜する押入れなどでは高いラドン濃度を示す。特にRC建物では1200Bq/m^3を越える場所がみられた。以上から、無機建材が用いられ換気が少なくなれば室内ラドン濃度が上昇することが明らかで、将来の日本の住宅もラドン対策が必要。